The purpose of the proposal is to determine the quantity and quality of the respiratory vagal afferent activity available to the newborn (NB) for the control of breathing. This aim consists of describing quantitatively the characteristics and properties of vagal (respiratory) slowly adapting (or stretch) and rapidly adapting (or irritant) receptors in the NB and growing dog as well as assessing the effectof CO2 on the discharge of slowly adapting receptors (SARs). Experiments will be performedon dogs from 1-30 days old. Animals will be anesthetized, paralyzed, and passively ventilated. Thin filaments will be teased from the cut right bafus nerve until action potentials from only 1 active fiber are obtained. The chest wil then be surgically opened. They types of vagal activity will be expressed as the percentage of fibers which are SARs and RARs. The distribution of SARs throughout the tracheobronchial tree will be obtained by localizing the receptor with direct probing of the airways and lung parenchyma and expressed as the percentage of SARs in the intra and extra thoracic trachea and the lung lobes. Vagal activity during the respiratory cycle will be represented as the percentage of SARs active at end expiration as well as during inflation versus the percentage of SARs active only during inflation. The transpulmonary pressure (Ptp stimulus) vs receptor dishcarge frequency (=response) will be obtained by inflating the lung to several predetermined Ptps and recording the receptor f. The shape of the curve (i.e. linear of plateauing) will be related to receptor location (i.e. tracheal or lobar) if possible since in the adult linear curves tend to be lobar receptors. The f at any given Ptp in the NB (1-5 d) will be compared to the adult and older animals. CO2 will be added to the inspired air to see if any inhibitory response is present on SARs of NB and young dogs. Receptor response will be related to location is possible (in the adult CO2 affects only lobar SARs and not tracheal SARs). The long term objective is to add to our knowledge of the normal physiology of the factors involved in the neonatal control of breathing. Furthermore, preliminary results suggest CO2 may stimulate SARs in NBs of a certain age thus increasing the inhibitory feedback to the respiratory centers, possibly limiting the NB's ability to respond to apneic or asphyxic periods.